Some of the books I stay away from for obvious reasons. Mildew/mold, stains and if you noticed books with the bar code on the inside front cover. Those tend to be fiction and little profit. Also, ones with the huge stickers on the back since those take too much time to take off.
What I tell my scouters to go is Scan first and if the book is flagged as a “BUY” then visually inspect the books. The visual inspection shouldn’t take more than 10 seconds but maybe for beginners it may take a little longer. I have them look at the Front and Back cover, inside front and back cover, spine, and open the book and flip through the pages to see of there is any highlighting, markings, underlining, etc.